From the book of rules (with added bolding):
14.3.4 Infants under two years and not occupying a seat, travel without the need to redeem Qantas Points as long as the accompanying adult is travelling on a Classic Flight Reward. There is a limit of one infant per adult travelling on a Classic Flight Reward. Taxes, fees and carrier charges (including GST) may be payable in relation to infants as specified by Qantas Loyalty at the time a Member books a Classic Flight Reward.
It looks like there’s provision for some extra charges, so I suggest you contact the call centre to get an idea of how it works (always keeping in mind that what one call centre agent tells you may not equal what another might say...)
I understand the rules for Qantas Classic Rewards when flying Qantas metal, just unsure what applies when using Qantas to book with other OW airlines.
For example, I'm flying Cathay HKG-PER later this year booked via Alaskan Mileage Plan. Alaskan cannot add an infant to the ticket, only Cathay can. Cathay charge 10% the full adult revenue fare ($600 one way!). If these rules applied to a 280k J award, i'd almost be better off saving an additional 280k points and booking the little one a seat. 10% of the full fare for multiple one-way flights in J would get very expensive
If because it is a Qantas award, booked via Qantas AND they can add the infant too all OW flights, then its a great deal (essentially the infant is free).